1. Field of the Invention
The present invention relates to the heat treatment of charges in a furnace and more particularly, the separation of charge holding devices being arranged in a row and being pushed in cyclic intervals through said furnace.
2. Prior Art
The operator of a heat treatment furnace often prefers to carry out a multi-stage heat treatment process in one and the same treatment chamber. In a process such as carburizing the charge is heated in a first stage, passes thereupon through a carburizing and diffusion treatment and is finally hardened. In said first heating stage, an atmosphere which does not produce sooting due to the relatively low temperature of the charge is desirable. In the carburization and diffusion zones of the furnace on the other hand, the carbon concentration of the furnace atmosphere should be at the maximum allowable level. In the hardening zone finally the charge is cooled und said cooling operation should preferably not be disturbed by heat transfer from the carburizing and the diffusion zones to the zone in which the charge is so cooled. Said zones are preferably isolated from each other. In the case of roller hearth furnaces of the type divulged, for example, in EP 0168788 each such heat treatment zone is provided with its own roller drive allowing the selective operation of the rollers in each such zone. With such an arrangement, the charges held in two adjacent zones may be moved relative to each other through selective roller drive operation and may be separated from each other by a door closing between two such zones. However, when compared with pusher type furnaces, roller hearth furnaces are associated with a number of drawbacks and disadvantages. If, for example, the rollers in such a furnace are exposed to a permanent load for a certain period of time they will tend to bend because of the relatively high temperature in the furnace chamber and must then be reversed and moved forward to prevent such bending. Further the cost of rollers and roller drives and the maintenance expense associated with rollers and roller drives are particularly high.
In the case of pusher type furnaces the separation of charge holding devices in the furnace chamber poses certain difficulties. If zones in the furnace are to be separated by doors then according to the state of the art, the direction of charge travel must be changed in conventional pusher type furnaces at the point of transition from one zone to the next following zone by the charge continuing its travel in the latter one of the two adjacent zones at a right angle to the direction of charge travel in the upstream zone. Furnaces in which the direction of charge travel is so changed require a relatively large space for installation.
The space required across the direction of charge travel is substantially smaller in the case of pusher type furnaces arranged in a line but prior art did not allow the isolation of furnace zones by doors. Known pusher type furnaces were therefore designed with a reduced cross section at the points of transition between adjacent furnace zones just allowing the charge to pass, but interference between two such adjacent zones had to be accepted.